Arthur/Dix: We X-rayed some MLB baseballs; here's what we found

From SABR member Rob Arthur and Tim Dix at FiveThirtyEight on March 1, 2018:

On 6,105 occasions last season, a major leaguer walked to the plate and hammered a baseball over the outfield wall. The 2017 season broke the home run record that was set in 2000 — the peak of the steroid era — when players hit 5,693 homers, and it built upon the remarkable 5,610 that were hit in 2016. It was a stunning display of power that played out in every MLB park almost every night. And with spring training underway in Florida and Arizona, MLB’s power surge is showing no sign of letting up.

But while we now know what caused the spike in home runs at the turn of the century — even if we didn’t at the time — the reason for the most recent flurry of long balls remains an unsolved mystery. Any number of factors might have contributed to the home run surge, including bigger, stronger players or a new emphasis on hitting fly balls. But none of those possibilities looms larger than the ball itself.

MLB and its commissioner, Rob Manfred, have repeatedly denied rumors that the ball has been altered in any way — or “juiced” — to generate more homers. But a large and growing body of research shows that, beginning in the middle of the 2015 season, the MLB baseball began to fly further. And new research commissioned by “ESPN Sport Science,” a show that breaks down the science of sports, suggests that MLB baseballs used after the 2015 All-Star Game were subtly but consistently different than older baseballs. The research, performed by the Keck School of Medicine at the University of Southern California and Kent State University’s Department of Chemistry and Biochemistry, reveals changes in the density and chemical composition of the baseball’s core — and provides our first glimpse inside the newer baseballs.